Détermination des paramètres de sorption sur sol de trois herbicides : oryzalin, isoxaben et norflurazon

Determination of sorption parameters on soil of three herbicides: oryzalin, isoxaben and norflurazon

Laboratoire des Sciences de l'Eau et de l'Environnement, Faculté des Sciences 123, avenue Albert-Thomas - 87060 Limoges Cedex, France

Résumé

En remplacement des triazines ou du diuron retrouvés fréquemment dans les eaux naturelles, trois nouveaux herbicides moins polluants (oryzalin, isoxaben, norflurazon) sont de plus en plus employés dans les vergers de pommiers en Limousin. Aussi, afin de caractériser leur comportement dans ce sol et de pouvoir les comparer aux précédents, des expériences d'adsorption de ces trois herbicides ont été réalisées en laboratoire sur deux horizons d'un sol de pommeraie. Les résultats montrent que les nouveaux herbicides sont mieux adsorbés par les couches supérieures du sol, couches riches en matière organique. La détermination des Koc, indice de mobilité, révèle que les herbicides de substitution aux triazines ou au diuron sont mieux retenus que ces derniers. Le calcul des indices G.U.S confirme ces résultats. Triazines et diuron sont des produits lessivables (G.U.S > 2,8). Isoxaben et norflurazon auraient un comportement intermédiaire (G.U.S moyen) alors que l'oryzalin apparaît comme presque non lessivable (G.U.S < 1,8).

Abstract

In Limousin, apple farming constitutes the main marketed vegetable production. This activity is also the largest consumer of pesticides. Applied directly on soil in pre or post growth, herbicides are easily carried away by run-off or percolation. Therefore, they can contaminate natural waters. A previous study, led on waters from apple orchard, showed that simazine and diuron are often found. So, to replace triazines and diuron, three new herbicides less toxic (oryzalin, isoxaben and norflurazon) are more and more employed by apple producers.

Adsorption batch experiments of these herbicides on two horizons of an apple orchard soil were performed in laboratory in order to characterize their behaviour in soil and to compare them to the previous pesticides. AU the residual concentrations of pesticide were measured out by H.P.L.C by detection to the optimal U. V wavelength of the product.

The results were introduced through adsorption kinetic and isotherm forms or interpreted according to the Freundlich model. The results point out that oryzalin, isoxaben and norflurazon are less persistent in soil than triazines or diuron but their degradation rate decreases with the depth of the loyer. Weed-killers are better adsorbed by the superior layers of the soil, layers rich in organic matter. The adsorption enhances with organic carbon content. Herbicides, for the most part, are retained by hydrogen bonds. Koc determination reveals that substitution weed-killers to the triazines or diuron are better fixed than these last compounds. Calculated values of the G.U.S confirm these results. Triazines and diuron are the most susceptible to migrate in soil (G.U.S > 2,8). Oryzalin is the best fixed and appears to be a non leachable compound (G.U.S < 1,8). Isoxaben has got an intermediate behaviour in inferior layer (1,8 < G.U.S < 2,8) but can be considered as a non moving compound on the layer A (G.U.S < 1,8). Norflurazon would have an intermediate behaviour whatever the layer tested is (1,8 < G.U.S < 2,8). These three herbicides are used on february or march and their ability to be fixed by soils increases their activity period and limits the contamination risks of the natural waters.